Restoring Nature's Coastal Architects: A Reality Check

Prior to the colonial settlement era, oysters were both physical (through the formation of reefs) and biological (through the structuring of food chains) architects of estuarine and coastal systems. Dunes and marshes at the land-water interface presented a complex system to guard the land margin from periodic storm inundation. Since the arrival of European colonists, a myriad of human mediated activities from direct shellfish harvest to environmental modification and degradation have resulted in the radically altered coastal and estuarine systems that we see today. Advancing knowledge of the role of natural structures such as reefs, dunes, and marshes play in stabilizing coastlines, facilitating ecosystem function, and maintaining species diversity has stimulated interest in and activity to restore them. In a world expectant of instant gratification and response, many such efforts have disappointed and achieved less than the desired end point. Why? Consider the time frames of formation of these natural structures, and the mechanisms of their maintenance once in place. Then it is obvious why that which was disassembled in decades will require comparable and longer time frames to rebuild, and that assumes many non-trivial biological challenges can be overcome as an integral part of the effort.

Consider the challenge of restoring reef-forming species. Oysters have been on the face of the planet with only modest modification in form for about 50 million years. Unlike the vast majority of the 8,000 or so species of closely related bivalves (shellfish with two valves or shells; think of the typical quahog you last ate at a raw bar) that approximate the shape of a laterally compressed egg and dig to live below the sediment-water interface, oysters have lost the ability to dig but perfected the trait of accreting reef structures above the sediment-water interface. This unusual ability, affected as each generation settles upon the generation before it, creates a physical structure that grows vertically in concert with sea level rise. The complex local architecture of the reef, in combination with tidal exposure, moderates predation loss of juvenile oysters while also providing a veritable condominium for many other small species that in turn support food chains culminating in apex predators (think of your favorite striped bass).

The landscape scale architecture of reefs within an estuary or coastal bay results from the interplay of bottom bathymetry, substrate accumulation (both shell and interleaving sediment), long term recruitment and growth of oysters, their eventual mortality that contributes to the shell base, and the loss of shell to burial, physical degradation and dissolution (the last being important in providing buffering against acidic conditions that will be exacerbated with climate change).

The dynamics of reef maintenance are clearly complex. Oyster growth and longevity provides a positive feedback for reef expansion through the supply of large shells to the structure upon oyster death. Conversely oyster mortality at a young age reduces shell input while harvest results in removal of shell from the accreting system—both contribute to negative feedback and reef degradation. Like reefs, the persistence of marshes is dependent on a positive feedback of sediment supply to fuel vertical accretion, while dunes deprived of sand supply are subject to structural failure. Feedback loops emerge as important quantitative processes that interact across this variety of shoreline protective structures. The magnitudes of response rates are comparable to that of sea level rise, that is, accretion rates of mm/y and over geological time frames support stable reefs, marshes and dune systems.

Destruction of natural reef and shoreline structures leaves human infrastructure vulnerable, but the expectation of restoration efforts producing Phoenix-like recoveries in sub-decadal time scales is unrealistic. Natural reefs are accreted over geological time frames. A reef at equilibrium is undergoing both accretive and loss processes as outlined above, and its form is the end point of many years of these positive and negative process—it is not an instantaneous result. Developing the complete suite of ecological services provided by intact reefs is a process of gradual increments. Human engineering options to protect shorelines have historically focused on hard structures devoid of the all important positive feedback properties...